This invention relates to gas relief valve designs and, more particularly, the invention is concerned with providing a gas relief valve design especially suitable for operation at cryogenic temperatures (-65.degree. F.) and contact forces where gas leakage would adversely affect the opening threshold pressure of the relief valve.
Heretofore, the operation of a gas relief valve subjected to high pressurization rates in a low temperature (-65.degree. F.) environment was generally erratic. Attempts to solve this problem were directed towards tighter dimensional tolerances and material revisions in the relief valve seal area. These attempts were not successful in eliminating erratic and unacceptable leakage at -65.degree. F. during the rapid pressurization of the valve.
The original configuration of the valve was inconsistent in operation at the -65.degree. F. environment. During activation of the valve, high pressure helium gas (3700-6000 psig) is discharged from a storage bottle into the container in which the relief valve is mounted. At the -65.degree. F. temperature using the original valve configuration, excessive amounts of gas were lost through the valve during the initiation transient (when the valve should have remained in the closed condition) with inadequate system pressures resulting. This problem resulted from gas leaking past the seat O-ring during initial pressurization of the O-ring at -65.degree. F. This is a common occurrence during low temperature sealing of gas. It is caused by thermal effects on the O-ring elastomer and lubricant, as well as distortion of the O-ring from its relaxed (unpressurized) condition to its final position against the downstream edge of the O-ring groove under full pressure load. This gas was trapped in the annulus formed by the metal-to-metal contact of the valve stem and container.
During normal operation, the contact force between the valve seat and the seal is provided by two sources: the wave spring force and a pressure-generated force. The pressure-generated force arises from a small circular area on the seat that is not subjected to inlet gas pressure. The remaining surface of the seat is pressure-balanced and does not produce any contribution to seat/seal contact forces. The seat/seal contact forces are approximately 3 lbs, and up to 30 lbs for the wave spring and pressure-generated forces, respectively. Any leakage gas trapped between the stem and container reduces the pressure-generated seat/seal contact force. When the seat/seal contact force is reduced sufficiently, gas can then flow by the seat/seal interface to increase the pressure on the end of the stem in contact with the container. This area is greater that the area of the valve seal with the result that the valve opens at less than 2/3 of the design cracking pressure. The hereinafter described invention will alleviate this problem and provide a tight seal which prevents the escape of helium gas during pressurization at -65.degree. F.